Propionic acid produced by Cutibacterium acnes fermentation ameliorates ultraviolet B-induced melanin synthesis

Ultraviolet irradiation induces melanin accumulation, which can be reduced by the use of chemical whitening products. However, the associated safety concerns of such products have prompted the search for natural and harmless alternatives. This study aimed to identify a natural acidic formulation to reduce skin pigmentation. The metabolite propionic acid (CH3CH2COOH, PA) was the most abundant fatty acid in the filtrate from Pluronic F68 (PF68) fermentation of Cutibacterium acnes (C. acnes) and reduced the DOPA-positive melanocytes by significantly inhibiting cellular tyrosinase activity via binding to the free fatty acid receptor 2 (FFAR2). Moreover, 4 mM PA treatment did not alter melanocyte proliferation, indicating that it is an effective solution for hyperpigmentation, causing no cellular damage. The reduced DOPA-positive melanocytes and tyrosinase activity were also observed in mice ear skin tissue injected with a mixture of C. acnes and PF68, supporting that the inhibition of melanogenesis is likely to be mediated through fermentation metabolites from C. acnes fermentation using PF68 as a carbon source. Additionally, PA did not affect the growth of its parent bacteria C. acnes, hence is a potent fermentation metabolite that does not disrupt the balance of the skin microbiome.

Application of transition metal ions in a study of photoinduced modifications of melanin

Short wavelength visible light is viewed as the main agent responsible for oxidative modification of melanin in the human retinal pigment epithelium (RPE). The aim of this research was to study light-induced modifications of melanin using iron and zinc as molecular probes. A synthetic model of eumelanin was treated by intense violet light. The interaction of melanin with metal ions was examined by electron paramagnetic resonance (EPR) spectroscopy and a thiocyanate assay. Weak photodegradation of melanin was shown to increase exposure of melanin subunits, while stronger photodegradation caused a loss of melanin subunits. Iron-binding in such melanin was weak and nonspecific.

Understanding the Role of Aggregation in the Broad Absorption Bands of Eumelanin

In this work, we investigate the relationship between the complex hierarchical assembly structure of eumelanin, its characteristic broad absorption band, and the highly unusual nonlinear dynamics revealed by pump-probe or transient absorption microscopy. Melanin-like nanoparticles (MelNPs), generated by spontaneous oxidation of dopamine, were created with uniform but adjustable size distributions, and kinetically controlled oxidation was probed with a wide range of characterization methods. This lets us explore the broad absorption bands of eumelanin models at different assembly levels, such as small subunit fractions (single monomeric and oligomeric units and small oligomer stacks), stacked oligomer fractions (protomolecules), and large-scale aggregates of protomolecules (parental particles). Both the absorption and pump-probe dynamics are very sensitive to these structural differences or to the size of intact particles (a surprising result for an organic polymer). We show that the geometric packing order of protomolecules in long-range aggregation is key secondary interactions to extend the absorption band of eumelanin to the low energy spectrum and produce drastic changes in the transient absorption spectrum.

Bio-Inspired Structural Colors Produced via Self-Assembly of Synthetic Melanin Nanoparticles

Structural colors arising from interactions of light with submicron scale periodic structures have been found in many species across all taxa, serving multiple biological functions including sexual signaling, camouflage, and aposematism. Directly inspired by the extensive use of self-assembled melanosomes to produce colors in avian feathers, we set out to synthesize and assemble polydopamine-based synthetic melanin nanoparticles in an effort to fabricate colored films. We have quantitatively demonstrated that synthetic melanin nanoparticles have a high refractive index and broad absorption spanning across the UV-visible range, similar to natural melanins. Utilizing a thin-film interference model, we demonstrated the coloration mechanism of deposited films and showed that the unique optical properties of synthetic melanin nanoparticles provide advantages for structural colors over other polymeric nanoparticles (i.e., polystyrene colloidal particles).

Iron species-mediated dopamine oxidation, proteasome inhibition, and dopaminergic cell demise: implications for iron-related dopaminergic neuron degeneration

Iron species have been suggested to be highly involved in the pathogenesis of Parkinson disease. However, the detailed mechanism of iron-induced dopaminergic degeneration is still unclear. In this study, we demonstrate that free iron ions (trivalent or bivalent) and iron ions in stable complex with cyanide ions (K(4)Fe(CN)(6) and K(3)Fe(CN)(6)) can induce dopamine (DA) oxidation with different profiles and subsequently lead to proteasome inhibition and even dopaminergic MN9D cell demise via different mechanisms. The free iron ions could mediate extensive DA oxidation in an iron-DA complex-dependent manner. However, iron ions in stable complex with cyanide ions could not induce, or could induce only brief, DA oxidation. Deferoxamine, a specific iron ion chelator, could disrupt iron-DA complex formation and thus abrogate free iron ion-catalyzed DA oxidation and subsequent cell toxicity. Glutathione could neither disrupt iron-DA complex formation nor influence free iron ion-catalyzed DA oxidation but could protect against iron-mediated toxicity via detoxification of toxic by-products of iron-mediated DA oxidation. The resulting DA oxidation could inhibit chymotrypsin-like, trypsin-like, and caspase-like proteasome activities. However, we demonstrated that oxidative damage was not the major toxic mechanism of MN9D cell degeneration, but it was the DA quinones derived from iron-induced DA oxidation that contributed significantly to proteasome inhibition and even dopaminergic cell demise.

Chemical and structural diversity in eumelanins: unexplored bio-optoelectronic materials

Eumelanins, the characteristic black, insoluble, and heterogeneous biopolymers of human skin, hair, and eyes, have intrigued and challenged generations of chemists, physicists, and biologists because of their unique structural and optoelectronic properties. Recently, the methods of organic chemistry have been combined with advanced spectroscopic and imaging techniques, theoretical calculations, and methods of condensed-matter physics to gradually force these materials to reveal their secrets. Herein we review the latest advances in the field with a view to showing how the emerging knowledge is not only helping to explain eumelanin functionality, but may also be translated into effective strategies for exploiting their properties to create a new class of biologically inspired high-tech materials.

Production of a novel neuromelanin at the sevoflurane-water interface

Postoperative cognitive dysfunction (POCD) occurs in the elderly following surgery that requires inhaled anesthetics. The molecular mechanism associated with this process is unknown. This study examined the possible role of serotonin, a neurotransmitter involved in cognition. We observed that sevoflurane, a common inhaled anesthetic, formed a separate phase in water similar to that of chloroform. Additionally, sevoflurane sequestered acrolein, which is a lipid peroxidation product associated with aging and is elevated in the elderly brain. The enhanced partitioning of acrolein increased the focal concentration and hence reactivity to serotonin which preferentially occurred at the sevoflurane-water interface. The resulting product exhibited unique properties similar to catecholamine-derived neuromelanin.

Amphetamine binding to synthetic melanin and scatchard analysis of binding data

Previous research into drug-hair binding shows that hair color affects drug-hair binding. There are no structural disparities in hair of different colors other than the type and content of melanin present. For this reason, this investigation focuses on synthetic eumelanin as a site for drug interaction using amphetamine as the candidate drug. The binding study was carried out at room temperature. The interaction between synthetic eumelanin and amphetamine was monitored using UV-Vis spectrophotometry at 257.2 nm. As the molecular weight of melanin is unknown, the number of binding sites could not be calculated directly. Hence the ratio of the number of mumoles of drug bound and the dry weight of melanin in mug was considered. Equilibrium was reached when approximately 32% of the drug was bound to melanin. Hence this study proves that amphetamine binds to synthetic eumelanin in vitro. Data interpretation using Scatchard analysis yielded a curvilinear plot with upward concavity indicating multiple binding sites on melanin and negative cooperativity.

Radiative Relaxation in Synthetic Pheomelanin

We report a detailed photoluminescence study of cysteinyldopa-melanin (CDM), the synthetic analogue of pheomelanin. Emission spectra are shown to be a far more sensitive probe of CDM's spectroscopic behavior than are absorption spectra. Although CDM and dopa-melanin (DM, the synthetic analogue of eumelanin) have very similar absorption spectra, we find that they have very different excitation and emission characteristics; CDM has two distinct photoluminescence peaks that do not shift with excitation wavelength. Additionally, our data suggest that the radiative quantum yield of CDM is excitation energy dependent, an unusual property among biomolecules that is indicative of a chemically disordered system. Finally, we find that the radiative quantum yield for CDM is approximately 0.2%, twice that of DM, although still extremely low. This means that 99.8% of the energy absorbed by CDM is dissipated via nonradiative pathways, consistent with its role as a pigmentary photoprotectant.

Mechanistic studies of melanogenesis: the influence of N-substitution on dopamine quinone cyclization

The influence of side-chain structure on the mode of reaction of ortho-quinone amines has been investigated with a view, ultimately, to developing potential methods of therapeutic intervention by manipulating the early stages of melanogenesis. Four N-substituted dopamine derivatives have been prepared and quinone formation studied using pulse radiolysis and tyrosinase-oximetry. Ortho-quinones with an amide or urea side chain were relatively stable, although evidence for slow formation of isomeric para-quinomethanes was observed. A thiourea derivative cyclized fairly rapidly (k = 1.7/s) to a product containing a seven-membered ring, whereas a related amidine gave more rapidly (k approximately 2.5 x 10(2)/s) a stable spirocyclic product. The results suggest that cyclization of amides, ureas and carbamates (NHCO-X; X = R, NHR or OR) does not occur and is not, therefore, a viable approach to the formation of tyrosinase-activated antimelanoma prodrugs. It is also concluded that for N-acetyldopamine spontaneous ortho-quinone to para-quinomethane isomerization is slow.

Quantitative fluorescence spectra and quantum yield map of synthetic pheomelanin

Spectroscopic studies of pheomelanin and its constituents have been sparse. These data present what is by far the most complete description of the fluorescence characteristics of synthetic pheomelanin. Emission spectra between 260 and 600 nm were acquired for excitation wavelengths between 250 and 500 nm at 1-nm intervals. A quantum yield map is also presented, correcting the fluorescence intensities for differences in species concentration and molar absorptivity. These fluorescence features exhibit interesting similarities and differences to eumelanin, and these data are interpreted with respect to possible chemical structures. Overall, these data suggest that pheomelanin oligomers may be more tightly coupled than those of eumelanin. Finally, the quantum yield is shown to be on the order of 10(-4) and exhibit a complex dependence on excitation energy, varying by a factor of 4 across the energies employed here.

Quantitative Fluorescence Excitation Spectra of Synthetic Eumelanin

Previously reported excitation spectra for eumelanin are sparse and inconsistent. Moreover, these studies have failed to account for probe beam attenuation and emission reabsorption within the samples, making them qualitative at best. We report for the first time quantitative excitation spectra for synthetic eumelanin, acquired for a range of solution concentrations and emission wavelengths. Our data indicate that probe beam attenuation and emission reabsorption significantly affect the spectra even in low-concentration eumelanin solutions and that previously published data do not reflect the true excitation profile. We apply a correction procedure (previously applied to emission spectra) to account for these effects. Application of this procedure reconstructs the expected relationship of signal intensity with concentration, and the normalized spectra show a similarity in form to the absorption profiles. These spectra reveal valuable information regarding the photophysics and photochemistry of eumelanin. Most notably, an excitation peak at 365 nm (3.40 eV), whose position is independent of emission wavelength, is possibly attributable to a 5,6-dihydroxyindole-2-carboxylic acid (DHICA) component singly linked to a polymeric structure.

High resolution magic angle spinning NMR spectroscopy used to investigate the ability of drugs to bind to synthetic melanin

Iodobenzamides are known to possess an affinity for melanoma tissue dependent on tumor pigmentation. In order to investigate the molecular interactions of drugs with melanin in vitro, a synthetic pigment swelled in deuterium buffer at physiological pH was used. The spectra of various mixtures of each Iodobenzamide (BZ) with melanin were studied at 25 degrees C by NMR under MAS conditions. The drug which interacts with the pigment exhibits linewidths greater than those observed for the free drug in solution. Line-broadening of the resonance occurred for the N-methyl group of acetylcholine or N-ethyl and aromatic groups of BZ. However, linewidths associated with methanol or hippuric acid were less altered by the presence of melanin. These observations indicate the specificity of the interaction between some drug moieties and the sites of melanin. From the concentration dependence of line-broadening, the apparent equilibrium dissociation constant (K(d)) of drug interaction with melanin was approached. It seems that the residual concentration-dependent line-broadening is caused by perturbations of ligand exchange between free and bound states and by differences in magnetic susceptibility present in the sample at the pigment-interacting drug moiety interface. Taken together, these results demonstrate the utility of this technique for investigating binding drugs.

Radiative relaxation quantum yields for synthetic eumelanin

We report absolute values for the radiative relaxation quantum yield of synthetic eumelanin as a function of excitation energy. These values were determined by correcting for pump beam attenuation and emission reabsorption in both eumelanin samples and fluorescein standards over a large range of concentrations. Our results confirm that eumelanins are capable of dissipating >99.9% of absorbed UV and visible radiation through nonradiative means. Furthermore, we have found that the radiative quantum yield of synthetic eumelanin is excitation energy dependent. This observation is supported by corrected emission spectra, which also show a clear dependence of both peak position and peak width on excitation energy. Our findings indicate that photoluminescence emission in eumelanins is derived from ensembles of small chemically distinct oligomeric units that can be selectively pumped. This hypothesis lends support to the theory that the basic structural unit of eumelanin is oligomeric rather than heteropolymeric.

Binding affinity of bunazosin, dorzolamide, and timolol to synthetic melanin

PURPOSE

The purpose of this study was to compare the binding affinity of bunazosin and dorzolamide to synthetic melanin relative to that of timolol.

METHODS

Synthetic melanin was prepared from dopa by the action of tyrosinase. Timolol, dorzolamide, and bunazosin were incubated separately at a concentration of 10(-4) M in 2 ml of 0.066 M phosphate buffer containing 5 mg of synthetic melanin. After centrifugation, the absorbance of each free drug in the supernatant was measured at its optimum wavelength. The percentage of each drug bound to melanin was calculated directly from the change in absorbance relative to the initial value.

RESULTS

The increase in the binding rates of all three drugs seemed to reach a plateau after 30 min. After incubating for 60 min, the binding rate of timolol was 22.2% +/- 4.9%, bunazosin 36.3% +/- 2.5%, and dorzolamide 8.5% +/- 1.9%. There were statistically significant differences between the binding rates of each drug.

CONCLUSIONS

Under our study conditions, the order of binding affinity of these ocular hypotensive agents to synthetic melanin seems to be as follows: bunazosin>timolol>dorzolamide.

Aluminum ions accelerated the oxidative stress of copper-mediated melanin formation

A comparison between the effects of aluminum and cupric ions on the dopachrome (DC) conversion and the cooperation effect of the both ions in the DOPA oxidation to melanin pathway has been studied by UV-Vis spectrophotometric method. Both aluminum and cupric ions catalyze the DC conversion reaction, which is an important step in the melanin synthesis pathway. However, cupric ions catalyze the conversion of DC to yield 5,6-dihydroxyindole-2-carboxylic acid (DHICA) but the product of DC conversion catalyzed by aluminum is 5,6-dihydroxyindole (DHI). DOPA oxidation catalyzed by aluminum and cupric ions is studied in the presence of hydrogen peroxide. The results from our experiments provide evidence that aluminum can markedly increase the oxidative stress of copper-mediated the melanin formation and influence the properties of the melanin by means of changing the ratio of DHICA/DHI in the acidic environment (pH 5.5).

Iron-binding characteristics of neuromelanin of the human substantia nigra

The vulnerability of the dopaminergic neurons of the substantia nigra (SN) in Parkinson's disease has been related to the presence of the pigment neuromelanin (NM) in these neurons. It is hypothesised that NM may act as an endogenous storage molecule for iron, an interaction suggested to influence free radical production. The current study quantified and characterised the interaction between NM and iron. Iron-binding studies demonstrated that both NM and synthetically-produced dopamine melanin contain equivalent numbers of high and low-affinity binding sites for iron but that the affinity of NM for iron is higher than that of synthetic melanin. Quantification of the total iron content in iron-loaded NM and synthetic melanin demonstrated that the iron-binding capacity of NM is 10-fold greater than that of the model melanin. This data was in agreement with the larger iron cluster size demonstrated by Mössbauer spectroscopy in the native pigment compared with the synthetic melanin. These findings are consistent with the hypothesis that NM may act as an endogenous iron-binding molecule in dopaminergic neurons of the SN in the human brain. The interaction between NM and iron has implications for disorders such as Parkinson's disease where an increase in iron in the SN is associated with increased indices of oxidative stress.

Interaction of quinidine, disopyramide and metoprolol with melanin in vitro in relation to drug-induced ocular toxicity

The aim of this work was to evaluate binding capacity of quinidine, disopyramide and metoprolol to melanin in vitro. The antiarrhythmics studied cause adverse reactions to the eye. Synthetic DOPA-melanin was used in the studies and a UV spectrophotometric method was employed to determine the drugs. The studies of the kinetics of the formation of quinidine-melanin, disopyramide-melanin and metoprolol-melanin complexes indicate that for all the complexes investigated the maximum time to reach reaction equilibrium is 24 h. Binding parameters, i.e., the numbers of independent binding sites and the association constants were determined on the basis of the Scatchard plots. An analysis of the binding curves obtained supports our conclusion that both strong (n1) and weak (n2) binding sites are involved in the formation of the complexes investigated. The total numbers of binding sites in synthetic DOPA-melanin complexes with quinidine, disopyramide and metoprolol were 0.525, 0.493 and 0.387 micromol/mg, respectively. The quinidine-melanin complex is characterized by greater stability (K1 = 3.00 x 10(5) M(-1), K2 = 1.75 x 10(3) M(-1)) in comparison with biopolymer complexes with disopyramide (K1 = 1.12 x 10(4) M(-1), K2 = 6.04 x 10(2) M(-1)) and metoprolol (K1 = 1.42 x 10(4) M(-1), K2 = 7.89 x 10(2) M(-1)). The ability of these drugs to form complexes with melanin in vitro may be one of the reasons for their ocular toxicity in vivo, as a result of their accumulation in melanin in the eye.

Inhibition of transglutaminase by synthetic tyrosine melanin

Transglutaminases catalyze the cross-linking and amine incorporation of proteins, and are implicated in various biological phenomena. Previously, we found a high molecular mass transglutaminase-inhibitory substance produced by Streptomyces lavendulae Y-200 that appeared to be a melanin substance. Here, we report that synthetic tyrosine melanin inhibited various types of transglutaminases. Tyrosine melanin inhibited tissue-type transglutaminase in a competitive manner with a glutamine substrate, and also inhibited the cross-linking of casein catalyzed by a tissue-type transglutaminase. The melanized hemolymph of the silkworm and melanin solutions prepared from melanin precursors inhibited tissue-type transglutaminase. These results suggested that the melanin substances generally inhibit transglutaminases.

Synthetic neuromelanin is toxic to dopaminergic cell cultures

In the present study, primary cultures of mesencephalic dopaminergic cells were exposed to synthetic dopamine neuromelanin (NM) for 48 hrs at concentrations of 0, 1, 10, 20, 50 and 100 microg NM/ml medium. Differently prepared synthetic NM with or without incorporated iron and NM oxidatively damaged by hydrogen peroxide were used. All NMs affected cellular structures e.g. as swelling of neural processes, rounding of cells, and occasional inclusion of neuromelanin particles. Cell numbers were uniformly and dose dependently reduced. Exposure to MPP(+) and ferric iron led to cytotoxic changes which could be further aggravated by oxidatively damaged NM, suggesting cytotoxicity of soluble compounds of NM in predamaged neurons.

Biomelanin antioxidants in cosmetics: assessment based on inhibition of lipid peroxidation

Acute adverse effects of ultraviolet (UV) radiation in humans include sunburn, photosensitivity reactions and immunological suppression. Chronic exposure to UV light, particularly the UVB (290-320 nm) component of the UV radiation, and certain environmental chemicals increase the risk of nonmelanoma skin cancer and play a major role in cutaneous aging. The lipid peroxidation (LPO) of biomembranes, mediated by reactive oxygen species and free radicals, is one of the major causes of cellular damage induced by UV radiation and toxins. Antioxidants, such as vitamin E, vitamin C and melanins, are reactive oxygen and radical scavengers, thereby minimizing the light- and toxin-induced tissue destruction. We examined the influence of 8 biotechnically produced polyphenolic melanins on the LPO of microsomal membranes in comparison with alpha-tocopherol, ascorbate and synthetic melanin. All biomelanins showed better inhibition of peroxidative damage than synthetic melanin. Three of the 8 tested drugs inhibited the LPO at least as effectively as vitamin C and vitamin E. The combination of the most effective biomelanin with both vitamin C and vitamin E resulted in greater LPO inhibition than caused by each agent alone. Our data show that biomelanins are potent inhibitors of the peroxidative destruction of biomembranes, indicating that these compounds may be useful antioxidative agents in cosmetic preparations.

Synthetic melanin suppresses production of proinflammatory cytokines

An overproduction of proinflammatory cytokines mediates the damaging sequelae of inflammation in pathologic conditions such as rheumatoid arthritis, graft-vs-host reaction, cachexia, and sepsis syndrome. We examined the cytokine regulatory activity of synthetic melanin, exemplified by biosynthetic l-glycine-l-tyrosine-based polymer (ME-1) and chemosynthetic dihydroxyphenylalanine-based polymer (MC-1). At nontoxic concentrations, both compounds effectively (>/=60%) and reversibly suppressed the production of tumor necrosis factor (TNF), even when applied after stimulation of human peripheral blood monocytes with lipopolysaccharide (LPS). The inhibitory activity of melanin was selective with regard to cytokine response but not inducer- or cell-type-specific. In addition to TNF, melanin inhibited production of interleukin (IL)-1beta, IL-6, and IL-10 but not granulocyte-macrophage colony-stimulating factor by the LPS-stimulated monocytes. Melanin was equally effective in inhibiting production of TNF by monocytes stimulated with the purified protein derivative of Mycobacterium tuberculosis and production of IL-6 by IL-1alpha-stimulated human fibroblasts and endothelial cells. Northern blot analysis, mRNA stability determination, immunoprecipitation studies on metabolically labeled intracellular TNF, and pulse chase experiments revealed that melanin reduced efficiency of mRNA translation. The finding that melanin arrests ongoing cytokine synthesis suggests that this compound may be useful as an adjunct therapy for conditions showing involvement of proinflammatory cytokines.

Femtosecond two-photon excited fluorescence of melanin

Fluorescence of synthetic melanin in dimethyl sulfoxide has been excited by two-photon absorption at 800 nm, using 120 fs pulses with photon flux densities > or = 10(27) cm-2 s-1. The shortest main component of the three-exponential decay of fluorescence is 200 +/- 2 ps. The overall spectral shape is red-shifted with respect to the 400 nm excited fluorescence. Two-photon excited melanin fluorescence also has been measured from excised samples of healthy human skin tissue. Because of the selectivity of melanin excitation via resonant two-photon absorption, it is hypothesized that fluorescence excited in this way may yield information on malignant transformation.

Pheomelanin as a binding site for drugs and chemicals

Certain drugs and chemicals, such as chloroquine, chlorpromazine, and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), are bound to melanin and retained in pigment cells for long periods. This specific retention in pigmented tissues can cause adverse effects in the skin, eye, inner ear, and pigmented nerve cells of the substantia nigra of the brain. To date, all studies have been focused on eu- and neuromelanin. In the present study, we show that chloroquine, chlorpromazine, chlomipramine, paraquat, acridine orange, and nickel, which are bound to eumelanin, also bind to synthetic pheomelanin, but the binding to pheomelanin is lower. The binding varied with the cysteine content and pH, and the results indicate that the binding is complex and includes ionic interactions. In addition, we have shown that these substances also bind to synthetic thiourea-containing melanin, but to quite a low extent. We also present a microautoradiographic study on the binding of 14C-chloroquine to natural pheomelanin in vivo in yellow mice C57BL (Ay/a). Black (C57/BL) and albino (NMRI) mice were used as controls. The autoradiography demonstrated a pronounced uptake of chloroquine in the hair follicles and the dermal melanocytes in the ear of yellow mice, which was comparable to the corresponding accumulation of label in black mice. In the albino mouse, the uptake was lower and more homogeneously distributed in the skin. These results suggest that the toxicological risks of melanin-related adverse effects are applicable to persons with a high content of pheomelanin in the skin and hair.

Solution structure of copper ion-induced molecular aggregates of tyrosine melanin

Melanin, the ubiquitous biological pigment, provides photoprotection by efficient filtration of light and also by its antioxidant behavior. In solutions of synthetic melanin, both optical and antioxidant behavior are affected by the aggregation states of melanin. We have utilized small-angle x-ray and neutron scattering to determine the molecular dimensions of synthetic tyrosine melanin in its unaggregated state in D(2)O and H(2)O to study the structure of melanin aggregates formed in the presence of copper ions at various copper-to-melanin molar ratios. In the absence of copper ions, or at low copper ion concentrations, tyrosine melanin is present in solution as a sheet-like particle with a mean thickness of 12.5 A and a lateral extent of approximately 54 A. At a copper-to-melanin molar ratio of 0.6, melanin aggregates to form long, rod-like structures with a radius of 32 A. At a higher copper ion concentration, with a copper-to-melanin ratio of 1.0, these rod-like structures further aggregate, forming sheet-like structures with a mean thickness of 51 A. A change in the charge of the ionizable groups induced by the addition of copper ions is proposed to account for part of the aggregation. The data also support a model for the copper-induced aggregation of melanin driven by pi stacking assisted by peripheral Cu(2+) complexation. The relationship between our results and a previous hypothesis for reduced cellular damage from bound-to-melanin redox metal ions is also discussed.

(D-(p-benzoylphenylalanine)13, tyrosine19)-melanin-concentrating hormone, a potent analogue for MCH receptor crosslinking

A photoreactive analogue of human melanin-concentrating hormone was designed, [D-Bpa13,Tyr19-MCH, containing the D-enantiomer of photolabile p-benzoylphenylalanine (Bpa) in position 13 and tyrosine for radioiodination in position 19. The linear peptide was synthesized by the continuous-flow solid-phase methodology using Fmoc-strategy and PEG-PS resins, purified to homogeneity and cyclized by iodine oxidation. Radioiodination of [D-Bpa13,Tyr19]-MCH at its Tyr19 residue was carried out enzymatically using solid-phase bound glucose oxidase/lactoperoxidase, followed by purification on a reversed-phase mini-column and HPLC. Saturation binding analysis of [125I]-[D-Bpa13,Tyr19]-MCH with G4F-7 mouse melanoma cells gave a K(D) of 2.2+/-0.2 x 10(-10) mol/l and a B(max) of 1047+/-50 receptors/cell. Competition binding analysis showed that MCH and rANF(1-28) displace [125I]-[D-Bpa13,Tyr19]-MCH from the MCH binding sites on G4F-7 cells whereas alpha-MSH has no effect. Receptor crosslinking by UV-irradiation of G4F-7 cells in the presence of [125I]-[D-Bpa13,Tyr19]-MCH followed by SDS-polyacrylamide gel electrophoresis and autoradiography yielded a band of 45-50 kDa. Identical crosslinked bands were also detected in B16-F1 and G4F mouse melanoma cells, in RE and D10 human melanoma cells as well as in COS-7 cells. Weak staining was found in rat PC12 phaeochromocytoma and Chinese hamster ovary cells. No crosslinking was detected in human MP fibroblasts. These data demonstrate that [125I]-[D-Bpa13,Tyr19]-MCH is a versatile photocrosslinking analogue of MCH suitable to identify MCH receptors in different cells and tissues; the MCH receptor in these cells appears to have the size of a G protein-coupled receptor, most likely with a varying degree of glycosylation.

A comparative study of the effect of pigment on drug toxicity in human choroidal melanocytes and retinal pigment epithelial cells

The aim of this study was to investigate whether the presence of pigment affects the sensitivity of pigmented cells of the eye, retinal pigment epithelium (RPE) and choroidal melanocytes (CMs) to the cytotoxic effects of xenobiotic drugs. Two approaches were used to compare pigmented versus unpigmented cells: RPE cells were repigmented by phagocytosis of synthetic melanin; UVB irradiation was used to induce an increase in pigment in both RPE and CMs. Three drugs known to induce toxicity in the eye, tamoxifen, chloroquine and thioridazine, were used to assess the sensitivity of cells to xenobiotic drugs. RPE cells were more resistant than CMs to the cytotoxic effects of all three drugs by a factor of 5-fold for tamoxifen, 7-fold for thioridazine and 30-fold for chloroquine. When RPE cells were repigmented using synthetic melanin, their sensitivity to tamoxifen was unchanged, they showed a slightly improved response to thioridazine (after 3 days of incubation with this drug), but they showed greatly increased toxicity to chloroquine (after 1 and 3 days of exposure to the drug), suggesting accumulation of this latter drug on the synthetic melanin. UVB irradiation was used to achieve an increase in the pigment content of both RPE and CMs. CMs were much more sensitive to UVB than RPE cells. CMs appeared to synthesise pigment via DOPA oxidase activity; RPE cells showed an increase in fluorescent material independent of any detectable DOPA oxidase activity. Irrespective of the nature of the pigment that UVB induced in melanocytes and RPE cells, their subsequent response to thioridazine and chloroquine was unchanged by the presence of this pigment.

Synthesis and characterization of new radioligands for the mammalian melanin-concentrating hormone (MCH) receptor

Melanin-concentrating hormone (MCH) is a neuropeptide present in the brain of all vertebrates. For the characterization of MCH receptors, a monoiodinated [Phe13, Tyr19]-MCH radioligand analogue was developed. The high susceptibility of [125I]-[Phe13, Tyr19]-MCH to oxidative damage and its very lipophilic nature made it necessary to develop new MCH radioligands. To increase the stability, native methionines were replaced by non-sulphur containing amino acid residues. In one analogue, the L-enantiomer of the phenylalanine residue at position 13 was substituted by the D-enantiomer, which increased the relative affinity of the ensuing [125I]-[D-Phe13, Tyr19]-MCH about 7-fold. The different analogues were iodinated by an enzymatic reaction and used for binding studies with mouse melanoma cells. [125I]-[Met(O)4,8, Phe13, Tyr19]-MCH and [125I]-[Hse4,8, Phe13, Tyr19]-MCH showed only about 19% of total binding and [125I]-[Ser4,8, Phe13, Tyr19]-MCH displayed about 44% of total binding when compared with [125I]-[Phe13, Tyr19]-MCH. Non-specific binding for all tracers was below 11% of total binding of [125I]-[Phe13, Tyr19]-MCH binding. [125I]-[D-Phe13, Tyr19]-MCH was used for saturation binding studies and revealed a KD of 122.7 +/- 15.3 pmol/l. This radioligand was further characterized by association and dissociation binding studies.

X-ray absorption fine-structure spectroscopy studies of Fe sites in natural human neuromelanin and synthetic analogues

X-ray absorption fine-structure spectroscopy is used to study the local environment of the iron site in natural (human) neuromelanin extracted from substantia nigra tissue and in various synthetic neuromelanins. All the materials show Fe centered in a nearest neighbor sixfold (distorted) oxygen octahedron; the Fe-O distances, while slightly different in the natural and synthetic neuromelanin, are both approximately 2.0 A. Appreciable differences arise, however, in the second (and higher) coordination shells. In this case the synthetic melanin has the four planar oxygens bound to carbon rings with Fe-C distances of approximately 2.82 and 4.13 A; the human sample does not show the 2.82 A link but instead indicates a double shell at approximately 3.45 and 3.78 A.

The incorporation of drugs into hair: relationship of hair color and melanin concentration to phencyclidine incorporation

Rodents with different hair pigmentation patterns were studied to evaluate the role of melanin in the incorporation of phencyclidine (PCP) into hair. There are two types of melanin in hair and other tissues: eumelanin, a brown-black pigment and pheomelanin, a reddish-yellow pigment. Sprague Dawley (SD; nonpigmented), Dark Agouti (DA; brown), Copenhagen (CP; brown hooded), Long Evans (LE; black hooded), and LBNF1 (deep brown) rats and Swiss-Webster (SW; nonpigmented), C57BL6 (black), and C57BL6 Ay/a (yellow) mice were administered PCP at 10 mg/kg/day for 5 days (n = 5 for each strain). Hair was collected either 14 (rats) or 35 (mice) days (mice) after beginning drug administration and analyzed for PCP, eumelanin, and pheomelanin. PCP concentrations in ng/mg (mean +/- SEM) were as follows: SD, 0.46 +/- 0.13; DA, 12.25 +/- 1.24; CP nonpigmented, 0.12 +/- 0.004; CP pigmented, 9.16 +/- 2.8; LE nonpigmented, 0.66 +/- 0.07; LE pigmented, 21.2 +/- 1.4; LBNF1, 21.64 +/- 3.8; SW, 0.48 +/- 0.36; C57 black, 11.0 +/- 4.03; and C57 yellow, 2.26 +/- 0.55. Eumelanin concentrations in microg/mg (mean +/- SEM) were as follows: DA, 20.50 +/- 1.58; CP pigmented, 19.43 +/- 0.40; LE pigmented, 17.56 +/- 0.61; LBNF1, 27.26 +/- 2.52; C57 black, 37.33 +/- 3.61; and C57 yellow, 1.76 +/- 0.02. Eumelanin was not detected in nonpigmented hair. Pheomelanin concentrations in microg/mg (mean +/- SEM) were as follows: DA, 0.09 +/- 0.00; CP pigmented, 0.20 +/- 0.03; LBNF1, 0.06 +/- 0.01; C57 black, 0.16 +/- 0.05; and C57 yellow, 29.16 +/- 0.97. Pheomelanin was not detected in nonpigmented or LE pigmented hair. These data demonstrate that PCP is incorporated into black hair to a greater extent than yellow or nonpigmented hair. There appears to be a linear relationship between the PCP concentration in hair and the ratio of eumelanin to pheomelanin. Our data suggest that despite variations in PCP concentration because of hair color, they may be normalized by using the ratio of eumelanin to pheomelanin rather than hair weight.

Hydrogen peroxide generation associated with the oxidations of the eumelanin precursors 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid

The ability of iron chelates to promote hydroxyl radical (.OH) formation from hydrogen peroxide (H2O2) via Fenton chemistry was exploited to detect H2O2 produced during the oxidations of the eumelanin precursors 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). H2O2 generation during the autooxidations of DHI and DHICA was confirmed on the basis of the electrochemical detection of three hydroxylation products of salicylate [2,3 and 2,5-dihydroxybenzoic acid (DHBA) and catechol], which was used as an .OH indicator. The oxidations of both 5,6-dihydroxyindoles were augmented by tyrosinase and peroxidase without the addition of H2O2. The partial inhibitions by catalase of the auto-oxidations and tyrosinase- and peroxidase-mediated oxidations of DHI and DHICA provide additional evidence of an endogenous origin of H2O2 during the final stages of eumelanogenesis. The mechanism proposed for the formation of H2O2 involves the semiquinones of DHI and DHICA in the univalent transfer of electrons to molecular oxygen. The observations described in this study support previous reports suggesting that factors modulating the levels of H2O2 in melanocytes and melanoma cells play critical roles in directing the course of melanogenesis and influencing the potential cytotoxicity of the biosynthetic pathways.

The effect of a synthetic neuromelanin on yield of free hydroxyl radicals generated in model systems

Neuromelanin is an amorphous pigment of the catecholamine origin that accumulates in certain dopaminergic neurons of the substantia nigra of human brain. In Parkinson's disease, there appears to be selective degeneration of the most heavily pigmented neurons of the substantia nigra, and this process has been linked to the presence of neuromelanin. It has been postulated that neuromelanin could increase the risk of oxidative stress reactions. On the other hand, melanin is usually considered to be an efficient antioxidant. Here we analyze experimental conditions that stimulate, or inhibit, antioxidant properties of neuromelanin. Using electron spin resonance (ESR)--spin trapping technique and salicylate hydroxylation assay, we monitored the formation of free hydroxyl radicals generated by a Fenton system in the presence of varying concentration of dopamine-melanin, a synthetic model for neuromelanin. Our data clearly indicate that the antioxidant action of neuromelanin is predominantly due to its ability to sequester redox-active metal ions such as iron. Using direct ESR spectroscopy, we have shown that ferric complexes with neuromelanin are resistant to reduction by mild biological reductants such as ascorbate. We have demonstrated that dopamine-melanin saturated with ferric ions, could enhance the formation of free hydroxyl radicals by redox activation of the ions. Thus, under the conditions that stimulate the release of accumulated metal ions, neuromelanin may actually become an efficient prooxidant. It is conceivable that neuromelanin, which normally is able to protect pigmented dopaminergic neurons against metal-ion related toxicity, could under extreme conditions have a cytotoxic role.

Synthesis and iodination of human (phenylalanine 13, tyrosine 19) melanin-concentrating hormone for radioreceptor assay

An analogue of human melanin-concentrating hormone (MCH) suitable for radioiodination was designed in which Tyr13 and Val19 of the natural peptide were replaced by phenylalanyl and tyrosyl residues: [Phe13, Tyr19]-MCH. The peptide was synthesized by the continuous-flow solid-phase methodology using Fmoc-strategy and polyhipe PA 500 and PEG-PS resins. The linear MCH peptides with either acetamidomethyl-protected or free cysteinyl residues were purified to homogeneity and cyclized by iodine oxidation, yielding the final product with the correct molecular weight of 2434.61. Radioiodination of the C-terminal tyrosine was carried out enzymatically using solid-phase bound glucose oxidase/lactoperoxidase, followed by purification on a reversed-phase mini-column and by high-pressure liquid chromatography. The resulting [125I]-[Phe13, Tyr19]-MCH tracer was the first radiolabelled MCH peptide suitable for radioreceptor assay: saturation binding analysis using mouse G4F-7 melanoma cells demonstrated the presence of 1090 MCH receptors per cell. The dissociation constant (KD) was 1.18 x 10(-10) M, indicating high-affinity MCH receptors on these cells. MCH receptors were also found in other cell lines such as mouse B16-F1 and G4F and human RE melanoma cells as well as in PC12 and COS-7 cells. Competition binding analyses with a number of other peptides such as alpha-MSH, neuropeptide Y, substance P and pituitary adenylate cyclase activating peptide, demonstrated that the binding to the MCH receptor is specific. Atrial natriuretic factor was found to be a weak competitor of MCH, indicating topological similarities between MCH and ANF when interacting with MCH receptors.

Binding sites for melanin-concentrating hormone (MCH) in brain synaptosomes and membranes from peripheral tissues identified with highly tritiated MCH

Melanin-concentrating hormone (MCH) is a neuropeptide occurring in the brain of all vertebrate species. In chromatophores of teleost fishes it induces pigment granule aggregation. In mammals, however, its physiological function is not yet clear. Attempts to identify the site(s) of its action by binding analysis failed because radioiodinated MCH with the natural sequence was devoid of biological activity. We have now synthesized an analogue of rat/human MCH, [Pra4,8,12,19]-MCH, containing four L-propargylglycine (Pra) residues in positions 4, 8, 12, and 19 for catalytic tritiation to norvaline ([3H4]Nva) residues, each of which containing four tritium atoms. The resulting [3H]-MCH ([(3H4)Nva4,8,12,19]-MCH) had a specific radioactivity of approx. 12,200 GBq/mmol (330 Ci/mmol) and retained a biological activity of 10% as compared to rat/human MCH when tested in the carp scale assay. A series of qualitative binding studies performed with rat crude membranes from brain and peripheal tissues as well as with rat brain synaptosomes using the [3H]-MCH radioligand provided the first evidence for the presence of MCH receptors in mammalian tissues. The data showed that specific binding is present in the hypothalamus, hippocampus and in the adrenal gland while none was detected in the brain cortex or spleen. Owing to the tendency of [3H]-MCH to non-specific binding to tissue, glass and plastic surfaces, a saturation binding analysis with this radioligand was not possible.

A 13C solid-state NMR study of the structure and auto-oxidation process of natural and synthetic melanins

This paper presents a 13C CP/MAS NMR study of the melanin pigments obtained through natural and synthetic origins: sepia-melanin from squid ink and three synthetic 5,6-dihydroxyindole-melanins prepared using different non-enzymatic oxidation pathways. The synthetic pigments can be distinguished from natural melanin by the absence of aliphatic carbons, thereby confirming the unreacted 3,4-dihydroxyphenylalanine and the proteinaceous origins of the aliphatic resonances in natural eumelanin. The spectra of selected non-protonated carbon resonances and those with only protonated carbon signals led to a quantitative analysis. An auto-oxidative experiment using a synthetic melanin, over a period of 130 h, has shown an unusually slow disappearance of hydrogen peroxide formed in situ. The 13C-NMR spectrum of the insoluble oxidized synthetic melanin compared to that before auto-oxidation clearly demonstrates that the oxidation process is associated with chemical changes within the pigment; i.e., carbonyl functional group formation and an increase of the non-protonated carbons fraction.

Melanized poly(HEMA) hydrogels: basic research and potential use

Synthetic melanogenesis, using epinephrine and other melanin precursors, within the matrix of hydrophilic polymers and copolymers of 2-hydroxyethyl methacrylate resulted in hydrogels able to absorb ultraviolet and visible radiation. This significantly enhances their value as materials for extraocular (contact lenses) or intraocular (artificial crystalline lenses) devices that should protect the retina of aphakic patients from potential damage induced by light. The two-phase morphology of melanized hydrogels, as investigated by TEM, revealed a fine structure that is possibly indicative of a true sequential interpenetrating polymer network. Their biocompatibility was evaluated by a set of different assays involving human choroidal fibroblasts. No cytotoxicity was found in the aqueous extracts of materials. By using an assay with cells and polymers embedded in a collagen gel, a short-range toxic effect was detected, presumably caused by melanin itself. However, in vivo experiments in animal eyes with melanized hydrogel intraocular lenses did not reveal any toxic reaction.

Application of photoacoustic phase angle spectroscopy (phi AS) to eumelanins and pheomelanins

A new technique, based on the measurement of the phase angle of the photoacoustic signal, was used for investigating the absorption spectra of melanins in the dry state. The main advantages of such a method are the insensitivity to scattering of the light and the applicability to nontransparent or highly adsorbing and thermally thick samples. The spectra obtained on different kinds of natural and synthetic melanins show much more details than the corresponding ones obtained spectrophotometrically on aqueous suspensions of the pigment. Some relevant features of the spectra still cannot be interpreted, yet we were able to show the presence in eumelanins of dopachrome at 475 nm and melanochrome at 540 nm, and in pheomelanins of compounds previously isolated from red chicken feathers. Moreover, the well known trend of the optical absorption, i.e., its decrease with increasing wavelength, was unambiguously confirmed.

A standardized test for the identification and characterization of melanins using electron paramagnetic resonance (EPR) spectroscopy

Melanins are complex, incompletely understood polymeric pigments that historically have been difficult to investigate with common chemical, histochemical, and physicochemical techniques. Because these pigments uniquely contain a stable population of organic free radicals, electron paramagnetic resonance (EPR) spectroscopy is a particularly effective method for studying them, and a set of qualitative EPR criteria has been established for their identification. However, a number of practical problems have arisen in applying these criteria to identify and characterize unknown pigments in relatively scarce pathological specimens, indicating that a standardized approach is needed. As reported here, a standardized EPR test for melanin based on the EPR criteria has been developed, guided by the requirements that it be sensitive, accurate, simple, and easy to interpret. It has been evaluated using the well-characterized synthetic melanin prepared by alkaline autooxidation of 5,6-dihydroxyphenylalanine (Dopa) and initially applied to the identification and characterization of an unknown pigment purified from an unusual malignant lung tumor.

Synthesis and characterization of melanins from dihydroxyindole-2-carboxylic acid and dihydroxyindole

Several studies have confirmed that a melanocyte-specific enzyme, dopachrome tautomerase (EC 5.3.2.3), catalyzes the isomerization of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) (Pawelek, 1991). Here we report that DHICA, produced either enzymatically with dopachrome tautomerase or through chemical synthesis, spontaneously polymerized to form brown melanin that was soluble in aqueous solutions above pH 5. Under the same reaction conditions, solutions of either DOPA, DOPAchrome, or 5,6-dihydroxyindole (DHI) formed black, insoluble melanin precipitates. When DHICA and DHI were mixed together, with DHICA in molar excess, little or no precipitation of DHI-melanin occurred and the rate and extent of soluble melanin formation was markedly enhanced over that achieved with DHICA alone, suggesting co-polymerization of DHICA and DHI. With or without DHI, DHICA-melanins absorbed throughout the ultraviolet and visible spectra (200-600 nm). The DHICA-melanins precipitated below pH 5, at least in part because of protonation of the carboxyl groups. DHICA-melanins could be passed through 0.22 micron filters but could not be dialyzed through semi-permeable membranes with exclusion limits of 12,000-14,000 daltons. HPLC/molecular sieve analyses revealed apparent molecular weights ranging from 20,000 to 200,000 daltons, corresponding to 100-1,000 DHICA monomers per molecule of melanin. DHICA-melanins were stable to boiling, lyophilization, freezing and thawing, and incubation at room temperature for more than 1 year. The natural occurrence of oligomers of DHICA was first reported by Ito and Nichol (1974) in their studies of the brown tapetal pigment in the eye of the sea catfish (Arius felis L.).(ABSTRACT TRUNCATED AT 250 WORDS)

Antioxidative activity of synthetic melanins. Cardiolipin liposome model

The inhibiting effect of melanin synthesized from dihydroxyphenylalanine (DOPA), dopamine, adrenaline and adrenolutin on the ultraviolet- or the Fe(2+)-ascorbic acid-induced peroxidation of cardiolipin liposomes has been studied. All these melanins are able to inhibit both the ultraviolet- and the Fe(2+)-ascorbic acid-induced lipid peroxidation. Antioxidative activity of melanins enhances in the order: dopamine-melanin less than melanin synthesized from dopamine in the presence of Cu(2+) less than DOPA--melanin less than melanin synthesized from adrenaline in the presence of Cu(2+) approximately equal to adrenolutin-melanin, and correlates with their ability to scavenge superoxide anion radical. The optical screening effect of the investigated melanins in the inhibition of lipid peroxidation was not higher than 15% for the most active melanins.

Synthesis and bioactivity studies of two isosteric acyclic analogues of melanin concentrating hormone

Salmon melanin concentrating hormone (MCH) is a cyclic heptadecapeptide. MCH stimulates perinuclear aggregation of melanosomes within integumental melanocytes of teleost fishes resulting in skin blanching. MCH contains a disulfide bridge forming a 10-residue ring [sequence: see text]. It has been proposed that the ring is necessary for maintenance of potency. In order to test this proposal, we have synthesized two pseudo-isosteric analogues of MCH that cannot cyclize. They differed only in the polarity of the side chain group of positions 5 and 14. Serine was substituted for Cys5 and Cys14 in one peptide and L alpha-aminobutyrate (Abu) was the substitution at the two positions in the other peptide. Using a fish skin bioassay we determined that these analogues exhibit less than 1/10,000th the potency of the native hormone. These results suggest that the disulfide bridge is necessary to maintain the correct conformational and topographical features of the hormone for receptor binding and transmembrane signal transduction.

"Black prosthesis" revisited: a study of epinephrine-induced pigment deposits on poly(methyl methacrylate)

To investigate in detail the "black prosthesis" syndrome, experimental production of melanin from epinephrine was performed both in bulk and onto the surface of a common prosthetic material, poly(methyl methacrylate) (PMMA). The study by ultraviolet/visible spectrometry showed that the radiation-absorptive properties of PMMA were significantly enhanced; a sample treated for 20 days in epinephrine absorbed all ultraviolet radiation up to a 344-nm wavelength and transmitted only 4.9% from the ultraviolet spectrum at 400 nm and 16.2% from the visible spectrum at 500 nm. Transmission electron microscopy studies suggest that melanogenesis occurs on the surface of PMMA, and the pigment does not penetrate the polymer. Using infrared spectrometry, it was confirmed that the pigmentation is caused by a melanin formed through the oxidative polymerization of epinephrine.

Structure-activity studies with fragments and analogous of salmonid melanin-concentrating hormone

A number of cyclic and linear fragments and analogues of MCH were synthesized and their biological potencies tested using the isolated carp scale melanophore assay. In this system the cyclic portion MCH(5-14) exhibited only 0.1% bioactivity, which was markedly enhanced by the addition of the exocyclic sequences MCH(15-17) and MCH(1-4). The exocyclic sequence itself, MCH(1-4,15-17), had minimal activity, however. Substitution of Tyr11 with phenylalanine reduced the potency of the ring structure MCH(5-14) by about 4-fold. Substitution of Gly8 with D-alanine reduced the potency of MCH(5-14) 16-fold, while both substitutions together caused a still more marked reduction (200-fold) in bioactivity. Linearized fragments of MCH, extending from MCH(15-17) to [Cys(Acm)5,14]MCH(1-17), showed a progressive increase in potency. The linearized forms of MCH, MCH(5-17) and MCH(5-14), were approximately 100-fold or less potent than their cyclic forms. The significant increases in bioactivity produced by the addition of the C- and N-terminal exocyclic sequence even to these linearized forms further emphasizes the importance of these regions for interaction at the receptor site.

X-ray diffraction studies on melanins in lyophylized melanosomes

A series of experiments was performed on lyophylized melanosomes in order to analyze the melanin in the natural state as polymerized into these organelles and to verify in such biological amorphous material the possibility of obtaining intensity scattering curves from which Bragg distances can be calculated. The results confirm the feasibility of the method and show that melanins in melanosomes maintain many structural features of the purified form and that the biochemical composition of the organelles can be responsible for the observed differences in the diffractograms. The presence in melanosomes of supramolecular paracrystalline aggregates was also clearly demonstrated.

Melanin-concentrating hormone: a structural and conformational study based on synthesis, biological activity, high-field NMR, and molecular modeling techniques

A series of Melanin-concentrating hormone (MCH) fragments have been synthesized and their biological activities compared with the parent peptide. The substructural units, 5-14 linear and 5-14 cyclic, have been used as models for MCH-- H-Asp1-Thr-Met-Arg-Cys-Met-Val-Gly-Arg HO-Val17-Glu-Trp-Cys-Pro-Arg-Tyr-Val in 1H-nmr conformational studies. Conformational features predicted by molecular dynamics analyses find support in the nmr experiments.

Diazoluminomelanin: a synthetic luminescent biopolymer

The purpose of this work was to synthesize a water-soluble derivative of 5-amino-2, 3-dihydro-1, 4-phthalazinedione (luminol) that generated sustained high level luminescence under physiologic conditions without the necessity of a catalyst. The derivative was made by a diazotization reaction with luminol and 3-amino-L-tyrosine. The resulting orange-brown anionic polymer has been given the trivial name of diazoluminomelanin (DALM). It was water soluble above and insoluble at or below pH 5.0. DALM luminesced when treated with hydrogen peroxide without the presence of a catalyst at pHs ranging from 6.5 to 12.0. Microgram quantities produced high levels of chemiluminescence for longer than 52 hr. Dried polymer generated a long-term stable electron spin resonance spectrum. The long-term chemiluminescence of DALM at pH 6.8-7.4 makes it a potentially useful reagent for detecting free radicals and peroxides in cellular and biochemical preparations.

Sources of the increased longitudinal relaxation rates observed in melanotic melanoma. An in vitro study of synthetic melanins

Researchers have suggested that the increased longitudinal relaxation rates (1/T1) of solvent water protons often found in melanoma result either from the paramagnetism of stable free radicals occurring in melanin or from that of methemoglobin in nonacute hemorrhagic regions of the tumor. However, field-cycling relaxometry and model solutions of synthetic melanin produced data which show that free radicals in melanin do not contribute significantly to 1/T1; instead, aggregation of melanin into macromolecular particles and binding of biologically-common paramagnetic metal ions (ie Fe3+, Mn2+, and Cu2+) to melanin effectively do increase 1/T1. These data have been combined with published histochemical data on melanin-containing tissues, while disregarding any additional effect related to hemorrhage. The result indicates that in melanoma the expected contribution of melanin-bound Fe3+ to 1/T1, at typical imaging fields, predominates under estimated in vivo conditions; furthermore, the total contribution from all sources, specifically due to the presence of melanin, is sufficient to account for reported measurements of 1/T1 in melanoma. Comparing the latter results with published data on T1 relaxation in model solutions of methemoglobin suggests that co-existing regions of nonacute microhemorrhage also may contribute significantly to 1/T1 under certain conditions. Finally, the implications for 1/T2 of melanin occurring in vivo within discrete melanosomes is discussed.

[Synthesis of eumelanin pigment precursors. II. 6-Hydroxy-5-methoxy- and 5-hydroxy-6-methoxyindol]

Following up of specific melanogenesis metabolite excretion in the course of malignant melanoma disease is useful for the disease prognosis assessment. The authors elaborated a modified synthesis technique of 6-hydroxy-5-methoxy and 5-hydroxy-6-methoxyindole, eumelanin pigment precursors. The elaborated synthesis is economically and temporally reasonable and the synthesized compounds accord, as proved by means of elementary analysis, thin layer (TLC) and high performance liquid chromatography (HPLC) and last but not least by nuclear magnetic resonance (NMR), with demands for reference compounds needed for isomer hydroxymethoxyindole quantification in urine.